That big asteroid was even bigger

A radar image from the Arecibo Observatory shows asteroid 2012 LZ1 from a distance of 6 million miles (10 million kilometers), at a resolution of 25 feet (7.5 meters) per pixel.

The bad news about the asteroid 2012 LZ1, which zipped past Earth last week, is that it's actually twice as wide and a lot deadlier than we thought — a kilometer (0.6 miles) wide in its largest dimension, rather than 500 meters. The good news is that we have at least seven centuries to figure out how to fight that particular space rock.

That's the verdict from astronomers using the 1,000-foot-wide (300-meter-wide) Arecibo Observatory in Puerto Rico, the world's biggest single radio dish.

"The sensitivity of our radar has permitted us to measure this asteroid's properties and determine that it will not impact the earth at least in the next 750 years," Mike Nolan, the observatory's director of planetary radar sciences, said in a news release issued today.

Another Arecibo researcher, Ellen Howell, was quoted as saying "this object turned out to be quite a bit bigger than we expected, which shows how important radar observations can be, because we're still learning a lot about the population of asteroids."

2012 LZ1 was discovered on June 10 at Siding Spring Observatory in Australia, and came within 14 Earth-moon distances (3.3 million miles, or 5.3 million kilometers) during its closest approach a week ago. There was zero risk of collision this time around, but the fact that astronomers had so little advance warning of LZ1's approach was just a bit, um, worrisome.

The big challenge for observing this asteroid appears to have been that it was unusually dark. That's why previous estimates of its size were so far off: Without precise observations of the object's shape, astronomers base their size estimates on the relative brightness of an asteroid at a given distance.

The Arecibo Observatory is well-suited for making radar observations of passing asteroids by reflecting radio signals off their surfaces — and the radar image of 2012 LZ1, captured on Tuesday, was good enough to show the object's shape and size. From that, scientists determined that the rock reflected only 2 to 4 percent of the light striking the surface. That suggests that the asteroid was as black or even blacker than charcoal.